Maximal Ca2+i stimulation of cardiac Na+/Ca2+ exchange requires simultaneous alkalinization and binding of PtdIns-4,5-P2 to the exchanger

Metabolic regulation of the squid nerve Na(+)/Ca (2+) exchanger: recent developments

In squid nerves, MgATP modulation of the Na(+)/Ca(2+) exchanger requires the presence of a cytosolic protein which becomes phosphorylated during the process. This factor has been recently identified. Mass spectroscopy and Western blot analysis established that it is a member of the lipocalin superfamily of lipid-binding proteins (LBP or FABP) of 132 amino acids. We called it regulatory protein of squid nerve sodium/calcium exchanger (ReP1-NCXSQ, access to GenBank EU981897).ReP1-NCXSQ was cloned, expressed, and purified. Circular dichroism, far-UV, and infrared spectroscopy suggest a secondary structure, predominantly of beta-sheets. The tertiary structure prediction provides ten beta-sheets and two alpha-helices, characteristic of most of LPB. Functional experiments showed that, to be active, ReP1-NCXSQ must be phosphorylated by MgATP, through the action of a kinase present in the plasma membrane. Moreover, PO4-ReP1-NCXSQ can stimulate the exchanger in the absence of ATP. An additional crucial observation was that, in proteoliposomes containing only the purified Na(+)/Ca(2+) exchanger, PO4-ReP1-NCXSQ promotes activation; therefore, this upregulation has no other requirement than a lipid membrane and the incorporated exchanger protein.Recently, we solved the crystal structure of ReP1-NCXSQ which was as predicted: a "barrel" consisting of ten beta-sheets and two alpha-helices. Inside the barrel is the fatty acid coordinated by hydrogen bonds with Arg126 and Tyr128. Point mutations showed that neither Tyr20Ala, Arg58Val, Ser99Ala, nor Arg126Val is necessary for protein phosphorylation or activity. On the other hand, Tyr128 is essential for activity but not for phosphorylation. We can conclude that (1) for the first time, a role of an LBP is demonstrated in the metabolic regulation of an ion exchanger; (2) phosphorylation of this LBP can be separated from the activation capacity; and (3) Tyr128, a candidate to coordinate lipid binding inside the barrel, is essential for activity.

Optimal metabolic regulation of the mammalian heart Na(+)/Ca(2+) exchanger requires a spacial arrangements with a PtdIns(4)-5kinase

In inside-out bovine heart sarcolemmal vesicles, p-chloromercuribenzenesulfonate (PCMBS) and n-ethylmaleimide (NEM) fully inhibited MgATP up-regulation of the Na(+)/Ca(2+) exchanger (NCX1) and abolished the MgATP-dependent PtdIns-4,5P2 increase in the NCX1-PtdIns-4,5P2 complex; in addition, these compounds markedly reduced the activity of the PtdIns(4)-5kinase. After PCMBS or NEM treatment, addition of dithiothreitol (DTT) restored a large fraction of the MgATP stimulation of the exchange fluxes and almost fully restored PtdIns(4)-5kinase activity; however, in contrast to PCMBS, the effects of NEM did not seem related to the alkylation of protein SH groups. By itself DTT had no effect on the synthesis of PtdIns-4,5P2 but affected MgATP stimulation of NCX1: moderate inhibition at 1mM MgATP and 1μM Ca(2+) and full inhibition at 0.25mM MgATP and 0.2μM Ca(2+). In addition, DDT prevented coimmunoprecipitation of NCX1 and PtdIns(4)-5kinase. These results indicate that, for a proper MgATP up-regulation of NCX1, the enzyme responsible for PtdIns-4,5P2 synthesis must be (i) functionally competent and (ii) set in the NCX1 microenvironment closely associated to the exchanger. This kind of supramolecular structure is needed to optimize binding of the newly synthesized PtdIns-4,5P2 to its target region in the exchanger protein.

Channelopathies linked to plasma membrane phosphoinositides

The plasma membrane phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) controls the activity of most ion channels tested thus far through direct electrostatic interactions. Mutations in channel proteins that change their apparent affinity to PIP2 can lead to channelopathies. Given the fundamental role that membrane phosphoinositides play in regulating channel activity, it is surprising that only a small number of channelopathies have been linked to phosphoinositides. This review proposes that for channels whose activity is PIP2-dependent and for which mutations can lead to channelopathies, the possibility that the mutations alter channel-PIP2 interactions ought to be tested. Similarly, diseases that are linked to disorders of the phosphoinositide pathway result in altered PIP2 levels. In such cases, it is proposed that the possibility for a concomitant dysregulation of channel activity also ought to be tested. The ever-growing list of ion channels whose activity depends on interactions with PIP2 promises to provide a mechanism by which defects on either the channel protein or the phosphoinositide levels can lead to disease.

Key role of a PtdIns-4,5P2 micro domain in ionic regulation of the mammalian heart Na+/Ca2+ exchanger

Phosphatidylinositol biphosphate (PtdIns-4,5P(2)) plays a key role in the regulation of the mammalian heart Na(+)/Ca(2+) exchanger (NCX1) by protecting the intracellular Ca(2+) regulatory site against H(+)(i) and (H(+)(i)+Na(+)(i)) synergic inhibition. MgATP and MgATP-gamma-S up-regulation of NCX1 takes place via the production of this phosphoinositide. In microsomes containing PtdIns-4,5P(2) incubated in the absence of MgATP and at normal [Na(+)](i), alkalinization increases the affinity for Ca(2+)(i) to the values seen in the presence of the nucleotide at normal pH; under this condition, addition of MgATP does not increase the affinity for Ca(2+)(i) any further. On the other hand, prevention of Na(+)(i) inhibition by alkalinization in the absence of MgATP does not take place when the microsomes are depleted of PtdIns-4,5P(2). Experiments on NCX1-PtdIns-4,5P(2) cross-coimmunoprecipitation show that the relevant PtdIns-4,5P(2) is not the overall membrane component but specifically that tightly attached to NCX1. Consequently, the highest affinity of the Ca(2+)(i) regulatory site is seen in the deprotonated and PtdIns-4,5P(2)-bound NCX1. Confirming these results, a PtdIns-5-kinase also cross-coimmunoprecipitates with NCX1 without losing its functional competence. These observations indicate, for the first time, the existence of a PtdIns-5-kinase in the NCX1 microdomain.

In the squid axon Na+/Ca2+ exchanger the state of the Ca i-regulatory site influences the affinities of the intra- and extracellular transport sites for Na+ and Ca2+

In squid axons, intracellular Mg2+ reduces the activity of the Na+/Ca2+ exchanger by competing with Ca2+ i for its regulatory site. The state of the Ca i-regulatory site (active-inactive) also alters the apparent affinity of intra- and extracellular transport sites. Conditions that hinder the binding of Ca2+ i (low pH i, low [Ca2+]i, high [Mg2+]i) diminish the apparent affinity of intracellular transport sites, in particular for Na i due to its synergism with H+ inhibition, but less noticeably for Ca2+ i because of its antagonism towards (Ha i + Na+ i) and Mg2+ i inhibitions. These are kinetic effects unrelated to the true affinity of the sites. With the Ca i-regulatory site saturated, the intracellular transporting sites are insensitive to [H+]i and to ATP. Likewise, the state of the Ca i-regulatory site (activated or inactivated) influences the affinity of the extracellular Ca o and Na o-transport sites (trans effects). In this case, the effects are opposite to those predicted by any of the transport schemes proposed for the Na+/Ca2+exchanger; i.e. its mechanism remains unexplained. In addition to their intrinsic importance for a full understanding of the properties of the Na+/Ca2+ exchanger, these findings show a new way by which the state of the Ca i-regulatory site may determine net movements of Ca2+ through this system.